From 04ca3a52f68275ce85355fb4c56f656080b20c92 Mon Sep 17 00:00:00 2001
From: Nick Mills-Barrett <nick@beeper.com>
Date: Thu, 9 Jun 2022 09:44:16 +0100
Subject: [PATCH] Use READ COMMITTED isolation level when inserting read
 receipts (#12957)

---
 changelog.d/12957.misc                     | 1 +
 synapse/storage/databases/main/receipts.py | 5 +++++
 2 files changed, 6 insertions(+)
 create mode 100644 changelog.d/12957.misc

diff --git a/changelog.d/12957.misc b/changelog.d/12957.misc
new file mode 100644
index 0000000000..0c075276ec
--- /dev/null
+++ b/changelog.d/12957.misc
@@ -0,0 +1 @@
+Use lower isolation level when inserting read receipts to avoid serialization errors. Contributed by Nick @ Beeper.
diff --git a/synapse/storage/databases/main/receipts.py b/synapse/storage/databases/main/receipts.py
index 21e954ccc1..b6106affa6 100644
--- a/synapse/storage/databases/main/receipts.py
+++ b/synapse/storage/databases/main/receipts.py
@@ -36,6 +36,7 @@ from synapse.storage.database import (
     LoggingTransaction,
 )
 from synapse.storage.engines import PostgresEngine
+from synapse.storage.engines._base import IsolationLevel
 from synapse.storage.util.id_generators import (
     AbstractStreamIdTracker,
     MultiWriterIdGenerator,
@@ -764,6 +765,10 @@ class ReceiptsWorkerStore(SQLBaseStore):
                 linearized_event_id,
                 data,
                 stream_id=stream_id,
+                # Read committed is actually beneficial here because we check for a receipt with
+                # greater stream order, and checking the very latest data at select time is better
+                # than the data at transaction start time.
+                isolation_level=IsolationLevel.READ_COMMITTED,
             )
 
         # If the receipt was older than the currently persisted one, nothing to do.